1. Field of the Invention
This invention relates to a developing apparatus and an image forming apparatus used in image formation by the electrophotographic process.
2. Related Background Art
A developing apparatus adopting a monocomponent contact developing process is known as a developing apparatus used in image formation by the electrophotographic process. This monocomponent contact developing process is such that a toner which is a monocomponent developer is carried on a developing roller as a developer carrying member, and is regulated and formed into a thin toner layer by a toner regulating blade, and a latent image on a photosensitive drum which is an image holding member (or image bearing member) is developed with the developing roller carrying the thin toner layer thereon brought into contact with the surface of the photosensitive drum.
The above-described monocomponent contact developing process is simple in apparatus construction and low in cost, as compared with a two-component developing process using a magnetic carrier besides a toner, and has the merit that it is easy to obtain uniform images low in edge effect, as compared with a noncontact process wherein development is effected with a predetermined spacing between a photosensitive drum and a developing roller, and is generally often used.
In the contact developing process, however, as compared with the noncontact developing process, the frictional contact between the photosensitive drum and the developing roller increases and therefore, as the number of image forming sheets increases, the deterioration of the toner by the frictional contact becomes greater. When the deterioration of the toner occurs, the formation of toner coagulations by the melting and close contact between the contacting surfaces of toner particles, as well as a reduction in the developing property, occurs, and the toner coagulations clog in the abutting surface between the developing roller and the toner regulating blade, and this had led to the problem that streak-like blanks are created in the thin toner layer to thereby bring about streak-like image deterioration (development streaks).
As a method of solving this problem, it is conceivable to burden, for example, a toner binder (binding resin) and prevent the coagulation (cohesion) of the toner, but it is difficult to keep balance between it and fixativeness. Or it is also conceivable to decrease the contact pressure between the photosensitive drum and the developing roller and the set pressure or the like of the toner regulating blade to thereby decrease the load to the toner, but this conversely results in a reduction in the frictional electrification capability for the toner and reduces the developing property, and this also suffers from the problem that it is difficult to keep balance in design.
We have variously investigated such problems to find that as the toner, a magnetic toner comprising a magnetic substance dispensed in a binder is very effective to prevent the development streaks caused by the melting and close contact of the toner. That is, it has been found that the dispersed magnetic substance protrudes on the surfaces of the toner particles and this works as a spacer and prevents the melting of and close contact between the toner particles and prevents the development streaks.
However, when a thin layer of magnetic toner is formed on the developing roller having no magnetic field producing means (magnet) therein, unevenness (coat unevenness) occurs to the thin toner layer and thus, the problem that unevenness appears on an image (unevenness of development) has become liable to arise. Moreover, it has been found that this phenomenon tends to occur more readily as the content of the magnetic substance becomes greater.
As the result of our investigation about this coat unevenness, we have come to think that in the toner, besides a toner charged to a regular polarity, a toner charged to the opposite polarity (reversal toner) is created, and this reversal toner and the regularly charged toner electrostatically attract each other to thereby reduce the fluidity of the toner and this is the cause of the toner coat unevenness.
When as is usual, magnetic field producing means such as a magnet is disposed in the developing roller, it is considered that the magnetic restraining force by the magnetic field producing means is stronger in the influence upon the toner coat than the electrostatic force with the reversal toner and therefore, it is difficult for the toner coat unevenness by the reversal toner to occur.
Japanese Patent Application Laid-Open No. 4-118678 discloses the use of a spherical magnetic toner in the contact developing process. In this publication, it is descried that for the prevention of fog, a magnetic field producing layer is provided in a developing roller to thereby restrain the magnetic toner on the developing roller side, but a method of improving the toner coat unevenness is not described clearly and moreover, the construction of the developing roller of this publication is complicated and results in a considerable increase in cost. Further, the spherical toner is difficult to manufacture stably and therefore is higher in cost than crushed toner.
Japanese Patent No. 2632053 discloses that in a developing apparatus using a nonmagnetic toner which is a monocomponent developer, a bias of ⅔ or less of a discharge starting voltage is applied to a toner regulating blade made of a metal to thereby prevent the electrical attraction of the toner to the regulating member by the action of an electric field and stabilize the toner coat state, thus expediting the frictional electrification of the toner and increasing the electrification among. However, this is a method only for increasing the total amount of toner electrification, and does not clearly describe a method of decreasing the reversal toner.
It is an object of the present invention to provide a developing apparatus and an image forming apparatus which make it possible to obtain good images by a contact developing method.
It is another object of the present invention to provide a developing apparatus and an image forming apparatus which makes it possible to stably coat a developer carrying member with a toner which is a monocomponent developer without causing streaks and unevenness and use the toner for development, and obtain good images free of streaks and unevenness.
It is still another object of the present invention to provide a developing apparatus comprising:
a developer carrying member carrying thereon a developer having a magnetic substance and carrying the developer, the developer carried on the developer carrying member contacting with an image holding member in a developing portion;
a regulating member for abutting against the developer carried on the developer carrying member and regulating the layer thickness of the developer carried on the developer carrying member; and
electric field forming means for forming an electric field between the developer carrying member and the regulating member, the intensity of the electric field formed between the developer carrying member and the regulating member being 10xc3x97106 to 28.3xc3x97106 (V/m).
It is yet still another object of the present invention to provide an image forming apparatus comprising:
an image holding member holding a latent image thereon; and
a developing apparatus for developing the latent image, the developing apparatus comprising:
a developer carrying member carrying thereon a developer having a magnetic substance and carrying the developer, the developer carried on the developer carrying member contacting with the image holding member in a developing portion;
a regulating member for abutting against the developer carried on the developer carrying member and regulating the layer thickness of the developer carried on the developer carrying member; and
electric field forming means for forming an electric field between the developer carrying member and the regulating member, the intensity of the electric field formed between the developer carrying member and the regulating member being 10xc3x97106 to 28.3xc3x97106 (V/m).
Other objects and features of the present invention will become more fully apparent from the following detailed description when read with reference to the accompanying drawings.